1. Field of the Invention
The present invention relates to a method of printing an image on a receiving medium by a printer comprising at least one print head with printing elements, each printing element associated with at least one compensating printing element, the image comprising a raster with rows of n pixels and columns of m pixels, each pixel having a value zero or an integer value greater than zero, the rows intended to be printed at a first printer resolution in the main scanning direction, the method comprising the step of assigning a printing element to each pixel of a part of the image, the part of the image comprising a plurality of rows of the image.
2. Background of the Invention
Printers, like inkjet printers and electrographic printers, are able to print an image on a receiving medium by means of the printing elements and comprise a processor unit for controlling print parameters with respect to printing the image and for performing calculations with respect to printing of the image.
In an inkjet printer with a print head, such print parameters may, for example, be a velocity of a carriage on which the print head is positioned, a jet frequency with which marking material drops are ejected from printing elements of the print head, a paper step in a sub-scanning direction, perpendicular to the main scanning direction, and a drop size of the marking material drops ejected from printing elements of the print head, if the printer has the capability of ejecting marking material drops of different drop sizes.
The processor unit performs calculations regarding the pixel information of the pixels of the image or a part of the image. The pixel may have a value zero indicating that the pixel is not intended to be printed, or an integer value greater than zero indicating that the pixel is intended to be printed.
The pixel having an integer value greater than zero may be printed by an ink drop with a drop size corresponding to the integer value. In the case of more drop sizes, more different values greater than zero may be used to indicate the drop size, for example, 1 (small drop size), 2 (middle drop size) and 3 (large drop size).
The processor unit also performs calculations regarding assignation of a printing element in the print head which is going to eject a marking material drop in order to print the pixel on the receiving medium. In the case of a printer using a plurality of process colors, a pixel has a value zero or an integer value greater than zero for each process color.
The processor unit determines a range of printer resolutions in a main scanning direction, while a predetermined printer resolution may be used in a sub-scanning direction. A printer resolution in one of the main- or sub-scanning directions is defined as a number of marking material drops to be ejected on the receiving material per length unit, for example 300 dpi (dots per inch). A printer resolution may also be expressed in a combination of the printer resolution of the main scanning direction and the printer resolution in the sub-scanning direction, for example 300 by 300 dpi or 300 by 2400 dpi. Such a printer resolution in either direction may be dependent on the printing parameters mentioned above. For example, if the velocity of the carriage is increasing, the printer resolution in the main-scanning direction becomes smaller. If the velocity of the carriage is decreasing, the printer resolution in the main-scanning direction becomes larger. If the jet frequency of the printing elements is increasing, the printer resolution in the main-scanning direction becomes larger. If the jet frequency of the printing elements is decreasing, the printer resolution in the main-scanning direction becomes smaller. Hereinafter we will presume that the marking material drop size is constant. However, the method may also be adapted to the use of marking material drops of different sizes.
The printing parameters may be tuned to obtain a printer resolution in the main-scanning direction out of the range of possible printer resolutions in the main-scanning direction. An engine of the printer may print the image on the receiving medium according to the values of the print parameters, resulting in a printed image with the desired printer resolution.
However, a problem may arise when a printing element becomes defective and does not eject a marking material drop any more. This may result in an artifact in the image, for example a white line in the image. Methods of camouflaging such artifacts, for example printing element failure correction methods, are known in the art. For example, neighboring printing elements may eject extra drops to compensate for the missing drops from the defective printing element. Printing element failure correction methods are applicable so that image information of a pixel that is assigned to a defective printing element is shifted to a nearby pixel position where it can be printed by a non-defective printing element.
In an ink jet printer, the print head of which comprises a plurality of nozzles as print elements, typically the nozzles are arranged in a line that extends in parallel with a direction (sub-scanning direction) in which a recording medium, e.g. paper, is transported through the printer, and the print head scans the paper in a direction (main scanning direction) perpendicular to the sub-scanning direction. In a single-pass mode, commonly a complete swath of the image is printed in a single pass of the print head, and then the paper is transported by the width of the swath so as to print the next swath or in general the single-pass mode is a mode wherein each position on the receiving medium to be covered by an ink drop according to the binary pixel information of the image is reachable only once by one nozzle. A pixel line in the binary pixel information may be printed by only one nozzle. In that case, when a nozzle of the print head is defective, e.g. has become clogged, the corresponding pixel line is missing in the printed image, so that image information is lost and the quality of the print is degraded.
A printer may also be operated in a multi-pass mode, in which only part of the image information of a swath is printed in a first pass and the missing pixels are filled-in during one or more subsequent passes of the print head. In the multi-pass mode, it is possible that a defective nozzle is backed-up by a non-defective nozzle, though mostly on the cost of productivity.
EP 1536371 discloses a method of the type indicated above, wherein, when a nozzle is defective, the print data are altered so as to bypass the faulty nozzle. This means that a pixel that would have, but cannot, be printed with the defective nozzle, is substituted by printing an extra pixel in one of the neighboring lines that are printed with non-defective nozzles, so that the average coverage of the image area is conserved and the defect resulting from the nozzle failure is camouflaged and becomes almost imperceptible. The method disclosed in EP 1536371 involves an algorithm that operates on a bitmap, which represents the print data, and shifts each pixel that cannot be printed to a neighboring pixel position.
However, such a camouflaging technique as described in EP 1536371 does not work sufficiently in the case of printing an image containing a high coverage area. In a high coverage area, all or nearly all pixel positions are intended to be printed with marking material from the printing elements of the print head. For example, in case of a solid part, the image coverage of that solid part is 100% and there are no pixel positions in neighboring lines which are available to cover up the image data for a pixel line to be printed by a defective printing element, because all of these pixel positions are already to be covered by marking material drops ejected from the other printing elements as being part of the high coverage area. Therefore, in a high coverage area, a small white line is visible in the printed image at the position of pixels of a defective printing element.
An example of such a situation is shown in FIGS. 3A-3C. A print head controller 24 or an image processor may address a nozzle to each pixel of a bitmap, for example a solid of 8 rows 31-38. Each row 31-38 of the solid is intended to be printed by a different nozzle out of the plurality of nozzles of the print head. When printing this solid of FIG. 3A on a receiving medium, ink drops 39 are ejected from the at least eight nozzles onto a part 300 of the receiving medium. The printed bitmap is shown in FIG. 3B. The ink drops 39 on the receiving medium are schematically represented by non-overlapping solid circles for convenience purposes. However, in practice, neighboring ink drops ejected on the receiving medium may partially overlap each other.
Upon detection of a defective nozzle, the rows which were to be printed by the defective nozzle are identified by the print head controller. For example, the third row 33 of the bitmap 25 was intended to be printed by the defective nozzle. However, the pixels of the third row 33 cannot be printed on the receiving medium, due to the defective nozzle. If the bitmap would be printed without any further image processing steps, the printed bitmap on the receiving medium would show up as shown in FIG. 3C. A white line 330 appears in the printed bitmap. Usually, a defective nozzle would be compensated by other nozzles. For example, the neighboring nozzles of the defective nozzle, which are addressed to the pixels of the second row 32 and the fourth row 34, could eject compensating ink drops to decrease the effect of the white line 330. The ink drops intended to be printed by the defective nozzle and by the compensating nozzles would cover a camouflaging area 340. Ink drops intended to be printed by the other nozzles would cover a two-part non-camouflaging area 350 outside the camouflaging area 340.
In order to eject extra compensating ink drops, the values of pixels in the second row 32 and/or pixels in the fourth row 34 should be turned from a value zero into an integer value greater than zero, in this case one. However, since there are no pixels 30 in the second row 32 and fourth row 34 which have a value zero, this is not possible. Moreover, since there are no pixels 30 in the solid at all which have a value zero, the defective nozzle cannot be compensated by means of extra ink drops by giving any other pixel 30 to be printed by other non-defective nozzles 31, 32, 34, 35, 36, 37, 38 a value of one. All the pixels 30 already have a value of one, so no extra ink drops can be generated by adapting any of the values of the pixels 30 of the bitmap 25.
In view of the above, especially when printing in a productive printing mode in which each pixel is only addressed once by a printing element, the defective printing element cannot be compensated by another printing element.